Don't worry about
it too much! The more you worry, the harder it's going to be. And it's not just going to leak out. You might feel like you need to poop, and if you can't hold it, just get up and head to the toilet. FWIW, I was in a pretty severe flare when I started using enemas. The first night, I only used half, per my GI's suggestion, and held it for at least 4 hours before I had to get up and use the toilet.
My BIGGEST tip is to squeeze the air out before you stick it up your bum and start squeezing. That extra air makes it a lot harder to hold it. A lot of people also warm the liquid in the bottle bit running it under some hot water or holding it in their hands, but I haven't needed to do that.
Status: Remission since May 2009!
Symptoms began in November 2008, ~4 weeks after giving birth to my son
Diagnosed with pancolitis on 1/30/09
Meds: Apriso (4 0.375g pills ONCE!! daily), mesalamine enema twice weekly, Natural Factors Ultimate probiotic 12/12 Formula, multivitamin. Used prednisone (starting dose 40 mg) to get into remission.